Time of flight (TOF) mass spectrometers are used to analyze the composition of a sample. The sample is ionized, accelerated through a vacuum, and caused to impact an ion detector. Ions having a higher mass accelerate more slowly through the vacuum than ions having a lower mass. As a result, the time of flight mass spectrometer measures the time of flight of the ions, which is then used to identify the mass of the ion. This information is then used to identify the content of the sample.
One type of detector used in TOF mass spectrometers includes an electron multiplier. The ions enter the electron multiplier and strike a dynode. In response, the dynode releases a plurality of electrons in response to each ion that strikes it. Those ions then pass to and strike another dynode. The second dynode then releases multiple electrons in response to each electron that strikes it. This process repeats for several stages of dynodes until enough electrons are generated to induce an electrical current. The current is measured and the time at which the current is induced corresponds to the time it took the ion to pass from the ion source to the electron multiplier.
A difficulty arises when the sample includes constituents that are not of interest for analysis. For example, some TOF mass spectrometers are commonly used to test the composition of a discrete sample of ambient environment such as air for the presence of any undesirable constituents such as pollution, poisons, and explosives. The instrument ionizes and samples all of the constituents that happen to be present, not just undesirable constituents that are of interest for analysis. However, the ionization and sampling includes high frequency and abundant molecules such as oxygen and nitrogen even though their presence is known and not of interest.
A difficulty is that the dynodes degrade with use, and frequently ionizing and sampling high abundance molecules that are not of interest shortens the dynode's useful life. One technique that has been used to prevent sampling ions that are not of interest is to add an arrangement of electrodes in the mass spectrometer that deflect the undesired ions from the ion path before they reach the electron multiplier. However, these arrangements are expensive, difficult to switch, consume energy, and add bulk to the mass spectrometer.